Frequency hopping is a commonly used technique for providing secure communication systems. A frequency hopping communication system is a type of spread spectrum system in which a wideband signal is generated by hopping from one frequency to another over a large number of frequency choices. In systems using very fast frequency hopping, the signal is transmitted at each frequency for a very short period, such as 20 ms, for example.
Transmission of frequency hopped signals include synch pulses and data pulses. The synch pulses allow a receiver to accurately adjust its local time to match the local time of the transmitter. For asynchronous communication systems, the hop pattern of the synch pulses is determined by the receiver before its clock can be adjusted. Once the hop pattern is determined, then the time-of-day can be extracted. The receiver typically uses non-coherent processing to perform time-of-arrival (toa) estimates on the synch pulses. The shape of the synch pulses is known, but the phase, carrier/offset frequency and timing are not known.
One approach is to use a matched filter for detecting the synch pulses. The matched filter operates based upon detecting the known shape of the synch pulses. Even though matched filters are straightforward to implement, there are several limitations. These limitations include signal-to-noise losses due to the frequency uncertainty of the synch pulses, and setting a noise-only threshold can be difficult when balancing the probability-of-detection (pod) to the probability-of-false alarms (pfa). In addition, time-of-arrival estimates of the synch pulses are based upon peak signal outputs. Since the synch pulses are typically at power levels lower than power levels of the data pulses, jammer or high noise level interference can mask the synch pulses, which in turn affects performance of the match filters.
Another approach for detecting the synch pulses is to use constant false alarm rate (CFAR) detectors. CFAR detectors maintain a fixed false alarm rate in the presence of changing interference levels. There are several different types of CFAR detectors, including cell-averaging (CA-CFAR), ordered-statistics (OS-CFAR), greatest-of (GO-CFAR) and censored (C-CFAR). A limitation of CFAR detectors is also with respect to a masking of the synch pulses due to threshold levels typically being higher than necessary, thus resulting in a reduced probability-of-detection.
Example receivers performing code synchronization using either matched filters or CFAR detectors are disclosed in several U.S. patents, such as U.S. Pat. No. 4,224,679; U.S. Pat. No. 6,757,546; U.S. Pat. No. 6,757,323; U.S. Pat. No. 6,226,321; U.S. Pat. No. 4,899,159; and U.S. Pat. No. 4,970,660.
As with matched filters, the need for increased threshold levels for CFAR detectors is due to inaccuracies in non-homogenous environments affected by antenna gain/noise temperature (G/T) variations and jammers. In addition, the signals containing the synch pulses may also fade in and out due to irregularities in the atmospheres. All of these factors contribute to decreasing the probability of an asynchronous receiver detecting received synch pulses.